<!DOCTYPE html>
<html lang="zh-CN">
<head>
    <meta charset="UTF-8">
    <title>声明变量</title>
</head>
<body>
<script>
    var myFirstVariable;
    myFirstVariable = 'Hello';
    alert(myFirstVariable);
    myFirstVariable = 54321;
    alert(myFirstVariable);
</script>
</body>
</html>